Optical switching device

ABSTRACT

An optical switching device includes a drive substrate, a transparent substrate, a liquid crystal layer, and a reflection enhancing film. The drive substrate includes a pixel region including a plurality of pixel electrodes, an outer circumferential region arranged at an outer circumference of the pixel region, and a seal region. The transparent substrate includes a counter electrode. The liquid crystal layer is interposed between the drive substrate and the transparent substrate. The reflection enhancing film is arranged on the pixel region, the outer circumferential region, and the seal region. The reflection enhancing film includes at least one assembly of dielectric films to be stacked, each assembly being a set of two dielectric films having different refractive indexes. The dielectric film as the first layer included in the reflection enhancing film has a different thickness from other dielectric films.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority under35 U.S.C. § 119 from Japanese Patent Application No. 2018-185093, filedon Sep. 28, 2018, the entire contents of which are incorporated hereinby reference.

BACKGROUND

The present disclosure relates to a structure of a reflection enhancingfilm in an optical switching device.

Ring-like optical network systems and optical wavelength multiplexsystems are recently proposed to deal with a markedly-increasing amountof information in the field of optical communication. Such an opticalcommunication system uses a reconfigurable optical add-drop multiplexer(ROADM) capable of branching or adding optical signals without beingconverted to electronic signals or translated.

An example of optical switching apparatuses used in a ROADM is awavelength selective switch (WSS). The WSS selects predeterminedwavelengths from an optical signal having a plurality of wavelengths,and allots the predetermined wavelengths to predetermined input/outputports so as to input/output multiplexed wavelengths. Examples of opticalswitching devices used in such a WSS include a micro electro mechanicalsystems (MEMS) mirror, and a liquid crystal on silicon (LCOS) device.

The LCOS device is a reflective liquid crystal device having a pixelregion in which a plurality of reflective pixel electrodes are arrangedin a horizontal direction and in a perpendicular direction. A refractiveindex of a liquid crystal on each pixel electrode varies when a voltageapplied to the liquid crystal is controlled per pixel electrode. A phasevelocity of a signal light is controlled per pixel such that therefractive index of the liquid crystal on each pixel electrode ischanged.

The LCOS device changes the phase velocity per pixel in a step wise, soas to control an inclination angle of a wave surface of the signal lightin accordance with a ratio of a change in the phase velocity. Namely,the LCOS device functions as a phase modulation device that changes thephase velocity per pixel to cause the signal light to be reflected in apredetermined direction.

The plural pixel electrodes are provided thereon with dielectric filmshaving different refractive indexes alternately stacked on one anotherto compose a reflection enhancing film for enhancing a reflectance dueto the effects of interference of light, so as to allow wavelengths ofthe signal light to be reflected effectively. Technology regardingreflection enhancing films is well known that alternately stacksdielectrics having a high refractive index and dielectrics having a lowrefractive index, each set to have a preferred thickness (opticalthickness=λ/4) at a targeted wavelength λ, so as to achieve the effectsof enhancing the reflection of light from the interfaces of therespective dielectrics due to the interference of light.

A conventional reflection enhancing film is formed such that lowdielectric films and high dielectric films each having a thicknesscorresponding to a wavelength of a signal light are stacked on oneanother. When a targeted wavelength of the signal light is changed, anew reflection enhancing film needs to be manufactured with a thicknesscorresponding to the changed wavelength.

JP 2008-158395 discloses an example of a phase modulation apparatususing an LCOS device. JP 2008-158395 discloses a method of fabricating areflection enhancing film by stacking high dielectric films and lowdielectric films on pixel electrodes of the LCOS device. This prior artdocument discloses a configuration of the reflection enhancing film withrespect to particular wavelengths, for example.

When the LCOS device is used as an optical switching device in a WSSdevice, the LCOS device reflects a signal light in a predetermineddirection by phase modulation. The LCOS device used as an opticalswitching device needs to use a liquid crystal layer having a thicknessof about 5 μm so as to ensure the phase modulation of 2π. The reflectionenhancing film on a plurality of pixel electrodes arranged on a drivesubstrate includes about five pairs of dielectric films (10 layers)stacked on one another, resulting in a thickness of about 0.5 μm.

The thicknesses of the high dielectric films and the low dielectricfilms composing the reflection enhancing film in the LCOS device areeach set to an optical thickness ((wavelength/4)/refractive index)capable of obtaining a reflected light due to the effects ofinterference of light most efficiently, depending on the wavelength ofthe signal light and the refractive index of each of the high dielectricfilms and the low dielectric films.

A step of forming the first layer in the reflection enhancing film isthe final step of a semiconductor manufacturing process of manufacturingthe drive substrate used in the LCOS device, and is typically undergonein a semiconductor wafer state. Subsequently, the rest of the dielectricfilms composing the reflection enhancing film are formed in the processof manufacturing the LCOS device. If a particular wavelength of pluralwavelengths of the signal light incident on the LCOS device is requiredto be reflected more efficiently, the thickness of the reflectionenhancing film needs to be changed. The drive substrate provided withthe first layer is hindered from film removal or a change in filmthickness through repairs, which is substantially unreasonable. Suchcircumstances typically lead the drive substrate to be disposed of, andhave an influence on a material loss and a period necessary for themanufacture of the LCOS device accordingly.

SUMMARY

According to an aspect of the embodiments, there is provided an opticalswitching device including: a drive substrate including a pixel regionincluding a plurality of pixel electrodes, an outer circumferentialregion arranged at an outer circumference of the pixel region, and aseal region; a transparent substrate including a counter electrode; aliquid crystal layer interposed between the drive substrate and thetransparent substrate; and a reflection enhancing film arranged on thepixel region, the outer circumferential region, and the seal region,wherein the reflection enhancing film includes at least one assembly ofdielectric films to be stacked, each assembly being a set of twodielectric films having different refractive indexes, and the dielectricfilm as a first layer in the reflection enhancing film has a differentthickness from other dielectric films.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an optical switching device according to thepresent embodiment showing a region for forming a reflection enhancingfilm.

FIG. 2 is a cross-sectional view showing an example of a structure ofthe optical switching device according to the present embodiment.

FIG. 3 is a cross-sectional view showing an example of a structure ofthe reflection enhancing film of the optical switching device accordingto the present embodiment.

FIG. 4 is a graph showing an example of spectral reflectancecharacteristics of the reflection enhancing film obtained in the presentembodiment.

DETAILED DESCRIPTION Embodiment

An optical switching device according to an embodiment of the presentdisclosure will be described below.

An example of a structure of the optical switching device is describedbelow with reference to FIG. 1 to FIG. 4. The present embodiment isillustrated with a LCOS device of a reflective liquid crystal deviceused as the optical switching device. As shown in FIG. 1 and FIG. 2, theoptical switching device 1 includes a drive substrate 10, a transparentsubstrate 20, a liquid crystal 31 filled into a liquid crystal layer 30,a seal material 42 including a spacer material 43 applied to a sealregion 40 and molded, and a sealant 50. The seal material 42 and thesealant 50 are a photo-curing resin, such as ultraviolet-curing resin.The seal material 42 and the sealant 50 may be either the same type ordifferent types of photo-curing resin.

The drive substrate 10 includes a pixel region 11, an outercircumferential region 18, and the seal region 40. The pixel region 11includes a plurality of light-reflective pixel electrodes 14 arranged inthe horizontal direction and in the perpendicular direction. A singlepixel electrode 14 corresponds to a single pixel. The outercircumferential region 18 surrounds the pixel region 11. The seal region40 surrounds the outer circumferential region 18.

A reflection enhancing film 17 is formed at least on the pixel region 11of the entire area including the pixel region 11, the outercircumferential region 18, and the seal region 40. An orientation film12 is formed at least on the reflection enhancing film 17. A pluralityof connection terminals 13 are formed at an outer circumferential partof the drive substrate 10.

The drive substrate 10 is a semiconductor substrate, in particular, asilicon substrate. The drive substrate 10 is provided with a drivecircuit (not shown) for driving the respective pixels under the pixelelectrodes 14. A material used for each of the pixel electrodes 14 andthe connection terminals 13 may be aluminum or an aluminum alloy, forexample.

The transparent substrate 20 includes a counter electrode 21 and anorientation film 22. The counter electrode 21 is arranged to correspondto the plural pixel electrodes 14. The orientation film 22 is formed onthe counter electrode 21. The drive substrate 10 and the transparentsubstrate 20 are arranged such that the plural pixel electrodes 14 andthe counter electrode 21 are opposed to each other.

The drive substrate 10 and the transparent substrate 20 are fixed by theseal material 42 and the sealant 50 with a gap GP provided therebetween.The liquid crystal layer 30 is provided in the gap GP between the drivesubstrate 10 and the transparent substrate 20. The liquid crystal layer30 is formed on an assembly of the plural pixel electrodes 14, thereflection enhancing film 17, and the orientation film 12. Anantireflection film 23 may be formed on the surface of the transparentsubstrate 20 opposite to the surface on which the counter electrode 21is provided. The antireflection film 23 may be a dielectric multi-layerfilm.

The transparent substrate 20, the counter electrode 21, and theorientation film 22 have light transmission properties. The transparentsubstrate 20 may be a non-alkaline glass substrate or a quartz glasssubstrate. A material used for the counter electrode 21 may be indiumtin oxide (ITO). A dielectric film having light transmission propertiesmay be provided on both of the upper and lower sides of the ITO film.

The seal material 42 is applied and molded along the outercircumferential region 18 which is an outer circumference of the pixelregion 11 so as to surround the pixel region 11 and the outercircumferential region 18. The seal material 42 is provided with aliquid crystal injection part 41. The liquid crystal layer 30 is formedsuch that the liquid crystal 31 is injected into the gap GP between thedrive substrate 10 and the transparent substrate 20 through the liquidcrystal injection part 41, and the liquid crystal injection part 41 issealed by the sealant 50. A thickness of the liquid crystal layer 30 inthe optical switching device 1 is 5 μm, for example.

A drive signal for driving the liquid crystal 31 is input to some of theconnection terminals 13. The drive circuit provided in the drivesubstrate 10 applies a drive voltage based on the drive signal to therespective pixel electrodes 14. The liquid crystal 31 is thus driven perpixel in accordance with a potential difference between the respectivepixel electrodes 14 and the counter electrode 21.

The structure of the reflection enhancing film 17 provided on the pixelregion 11, the outer circumferential region 18, and the seal region 40is described in detail below. Since the plural pixel electrodes 14 arearranged in the pixel region 11 indicated by the oblique lines in FIG.1, the pixel region 11 has an uneven surface with projections when onlyprovided with the pixel electrodes 14. As shown in FIG. 3, thereflection enhancing film 17 is formed such that dielectric filmsincluding dielectrics (for example, silicon dioxide: SiO₂) are stackedon the pixel region 11, the outer circumferential region 18, and theseal region 40.

First, a SiO₂ film (L1) as the first film is formed on the pixel region11, the outer circumferential region 18, and the seal region 40 byvacuum vapor deposition, for example. The SiO₂ film provided is a thinfilm having an uneven surface with projections conforming to the unevensurface of the pixel region 11 (the lower layer) only provided with theplural pixel electrodes 14. The first layer LI may be formed in a waferstate in the middle of the semiconductor manufacture process ofmanufacturing the drive substrate 10.

The film obtained is flattened by chemical mechanical polishing (CMP) soas to have an even surface after the film formation. The flatteningprocess is not limited to the chemical mechanical polishing, and may beperformed by chemical etching, for example. The flattening process maybe performed in a semiconductor wafer state.

The flattening process after the formation of the first layer (L1) ofthe reflection enhancing film 17 is typically performed in a state of asemiconductor wafer in which plural drive substrates 10 are arranged.The reason for this is that the flattening process performed in thestate of the semiconductor wafer in which the plural drive substrates 10are arranged on the same plane, preferably avoids unevenness of aflattened level per drive substrate 10 more reliably than the processingof flattening performed on an individual chip.

Subsequently, a dielectric having a higher refractive index than SiO₂ isstacked on the flattened top surface of the SiO₂ film (L1) by vacuumvapor deposition, for example. An example of material having a higherrefractive index than SiO₂ is silicon nitride (Si₃N₄). The surface ofthe Si₃N₄ film (L2) conforms to the surface configuration of the SiO₂film, which is flat without projections. Another example of materialhaving a higher refractive index than SiO₂ may be titanium dioxide(TiO₂) or tantalum pentoxide (Ta₂O₅). Alternatively, the material may bedetermined as appropriate depending on the wavelength of the signallight incident on or reflected off the LCOS device or the requiredcharacteristics.

An assembly of dielectric films, which is a set of a low dielectric filmof SiO₂ (L1) and a high dielectric film of Si₃N₄ (L2), is repeatedlystacked from L1 to L10 to form the reflection enhancing film 17. Thetenth layer (L10) in the reflection enhancing film 17 has a flattenedsurface, not conforming to the uneven surface of the pixel region 11(the lower layer) only provided with the pixel electrodes 14, since thesurface of the SiO₂ film as the first layer (L1) is flattened. While thepresent embodiment is illustrated with the reflection enhancing film 17including the five assemblies of the dielectric films from L1 to L10,the number of the stacked layers may be determined as appropriatedepending on the preferred reflectance.

As an example of the thicknesses of the dielectric films in theassembly, when a targeted wavelength λ₀ for increasing the reflectanceis 350 nm, the thickness of the low dielectric film of SiC₂ is(λ/4)/nd1=(350/4)/1.48=59.10 nm when the refractive index nd1 of the lowdielectric film of SiO₂ is 1.48, and the thickness of the highdielectric film of Si₃N₄ is (λ/4)/nd2=(350/4)/2.10=41.60 nm when therefractive index nd2 of the high dielectric film of Si₃N₄ is 2.10.

With regard to the number of the stacked dielectric films included inthe reflection enhancing film 17, five or more assemblies of thedielectric films (ten or more layers) are used so as to achieveparticularly high reflection enhancing effects. The total thickness forachieving the reflection enhancing effects with the signal lightincluding a visible range of wavelengths needs to be 0.5 μm or greater.For example, when five assemblies of the dielectric films are stacked,and each assembly is a set of the SiO₂ film having a thickness of 94 nmand the Si₃N₄ film having a thickness of 65 nm so as to enhance thereflectance in the band of green of 550 nm, the total thickness of thefilm formed is about 0.8 μm.

In particular, the SiO₂ film as the first layer (L1) in the reflectionenhancing film 17 is formed having a thickness of λ/4 of an intermediatewavelength λ_(c) from the shortest wavelength to the longest wavelengthin the wavelength band of the signal light SL. For example, when theintermediate wavelength λ_(c) is 550 nm, the thickness of the SiO₂ filmas the first layer (L1) is set to result in 75 nm after the flatteningprocess.

Subsequently, four assemblies of the dielectric films (four sets: a setof L3 and L4, a set of L5 and L6, a set of L7 and L8, and a set of L9and L10), each including the low dielectric film of SiO₂ and the highdielectric film of Si₃N₄, are sequentially stacked from L3 to L10 on thetop surface of the assembly of the dielectric films as a set of the SiO₂film (L1) and the Si₃N₄ film (L2) subjected to flattening, so as to formthe reflection enhancing film 17 in the same manner as the conventionalcase. While the present embodiment is illustrated with the thin film ofthe reflection enhancing film 17, in which the five assemblies of thedielectric films are stacked, the number of the stacked layers may bedetermined as appropriate depending on the preferred reflectance.

As a comparative example, the conventional case uses a reflectionenhancing film including layers each having a thickness according to atargeted wavelength λ₀. In particular, when the targeted wavelength λ₀of the signal light SL is 350 nm, the thickness of the SiO₂ film as thefirst layer of the reflection enhancing film 17 is 59 nm so as tocorrespond to the wavelength of 350 nm. Thereafter, the five assembliesof the dielectric films are stacked together while the thickness of theSi₃N₄ films of the high dielectric films is set to 41 nm, and thethickness of the SiO₂ films of the low dielectric films is set to 59 nm.In this case, the entire reflection enhancing film 17 can achieve areflectance of 98% at the wavelength λ₀ of 350 nm of the signal lightSL.

If the targeted wavelength λ₀ is changed from 350 nm to 550 nm in theconventional case, the SiO₂ film as the first layer (L1) needs to beformed to have a thickness of 94 nm during the semiconductor manufactureprocess. A high reflectance cannot be achieved unless the thickness ofthe Si₃N₄ films of the high dielectric films is changed to 65 nm and thethickness of the SiO₂ films of the low dielectric films is changed to 94nm in the following layers from the second layer (L2).

In contrast, since the SiO₂ film as the first layer (L1) according tothe embodiment of the present disclosure has already been subjected toflattening so as to have a thickness set to 75 nm in accordance with theintermediate wavelength λ_(c) of 550 nm, the thickness of the Si₃N₄films of the high dielectric films and the thickness of the SiO₂ filmsof the low dielectric films are only required to be changed to 65 nm and94 nm, respectively, in the following layers from the second layer (L2),so as to achieve 96% of the reflectance of the entire reflectionenhancing film 17. FIG. 4 shows spectral reflectance characteristics ofthe reflection enhancing film actually manufactured. Namely, a highreflectance can be achieved without the thickness of the first layer(L1) changed.

Setting the thickness of the first layer (L1) (for example, 75 nm) inaccordance with the intermediate wavelength λ_(c) in thepreliminarily-presumable band of the signal light SL in the band fromnear ultraviolet light to near infrared light, can provide thereflection enhancing film 17 having a high reflectance by stacking thelow dielectric films and the high dielectric films in the followinglayers from the second layer, each having a thickness set in accordancewith the targeted wavelength λ₀, regardless of whether the targetedwavelength λ₀ is changed.

The present embodiment can be applied to a case in which the signal bandof the signal light SL is an infrared band of 1200 nm to 1700 nm. Inparticular, the thickness of the low dielectric film of SiO₂ as thefirst layer (L1) of the reflection enhancing film 17 in which theintermediate wavelength λ_(c) is 1450 nm, may be set to(1450/4)/1.48=244 nm. Thereafter, the high dielectric films and the lowdielectric films with the respective thicknesses set in accordance withthe targeted wavelength λ₀ may be stacked on one another.

According to the structure of the reflection enhancing film 17 of theoptical switching device according to the embodiment, the dielectricfilm as the first layer has a thickness (optical thickness λ/4) set toachieve the reflection enhancing effects at about the intermediatewavelength in the presumable wavelength band of the signal light SL, sothat the thicknesses of the dielectric films after the second layer caneach be set in accordance with the targeted wavelength. The reflectionenhancing film 17 having a high reflectance in the targeted band can beformed accordingly. In other words, stacking the dielectric films of thereflection enhancing film 17 after the second film with the thicknessesset in accordance with the targeted wavelength λ₀, facilitates theformation of the reflection enhancing film 17 having a high reflectance,even if the targeted wavelength λ₀ is changed, so as to preventdegradation of the performance of the optical switching device.

The structure of the reflection enhancing film 17 in the opticalswitching device according to the embodiment can prevent scattering ordiffraction of the signal light SL incident on or reflected off the LCOSdevice due to the flattened surface of the reflection enhancing film 17,so as to achieve the reflective characteristics of the reflectionenhancing film 17 Identical to an optical simulation, preventingdegradation of the performance of the optical switching device.

It should be understood that the present disclosure is not intended tobe limited to the above embodiment, and various modifications can bemade within the scope of the present disclosure. For example, a changein the number of the sets of the dielectric films to ten (twenty layers)may be made depending on the required characteristics in order to allowthe signal light to be reflected in a narrower band.

The LCOS device, when used as the optical switching device according tothe present embodiment, can easily ensure the reflection enhancing filmhaving a high reflectance, even if a targeted wavelength of a signallight is changed, so as to keep the performance of the optical switchingdevice.

What is claimed is:
 1. An optical switching device comprising: a drivesubstrate including a pixel region including a plurality of pixelelectrodes, an outer circumferential region arranged at an outercircumference of the pixel region, and a seal region; a transparentsubstrate including a counter electrode; a liquid crystal layerinterposed between the drive substrate and the transparent substrate;and a reflection enhancing film arranged on the pixel region, the outercircumferential region, and the seal region, wherein the reflectionenhancing film includes at least one assembly of dielectric films to bestacked, each assembly being a set of two dielectric films havingdifferent refractive indexes, and the dielectric film as a first layerin the reflection enhancing film has a different thickness from otherdielectric films.
 2. The optical switching device according to claim 1,wherein the respective dielectric films having an identical refractiveindex have an identical thickness except for the dielectric film as thefirst layer included in the reflection enhancing film.
 3. The opticalswitching device according to claim 1, wherein the dielectric film asthe first layer included in the reflection enhancing film is subjectedto flattening.